Multi-bit magnetic memory cell

ABSTRACT

Apparatus includes a first Hall sensor having a first terminal, a second terminal, a third terminal and a fourth terminal and a second Hall sensor having a fifth terminal, a sixth terminal, a seventh terminal and an eighth terminal. A conductor connects the third terminal to the fifth terminal. A processor is configured to measure a first potential between the fourth terminal and the sixth terminal while transferring a first current from the first terminal to the seventh terminal via the conductor, to measure a second potential between the first terminal and the seventh terminal while transferring a second current from the fourth terminal to the sixth terminal via the conductor, and to determine a resultant voltage generated by the first and second Hall sensors in response to the first and second potentials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/235,133, filed Jan. 27, 2014, in the national phase of PCT PatentApplication PCT/IB2012/053674, filed Jul. 19, 2012, which claims thebenefit of U.S. Provisional Patent Application 61/514,064, filed Aug. 2,2011, whose disclosure is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to magnetic memory devices, andparticularly to memory devices where data stored in magnetic media isread using the extraordinary Hall effect.

BACKGROUND OF THE INVENTION

Non-volatile magnetic random access memories (MRAMs) have been proposedas candidates to replace conventional dynamic random access memories(DRAMs) and hard disk drives. Such memory devices make use of the giantmagnetoresistance (GMR) and tunneling magnetoresistance (TMR) sensingtechniques. Currently implemented MRAM devices utilize two magneticlayers magnetized in-plane or out-of-plane so that variations in the GMRand/or TMR may be measured. The magnetic layers have magneticorientations which are either parallel or anti-parallel to each other,which create four possible magnetic states and give rise to twodifferent GMR or TMR resistances associated with bits “0” and “1”.

A different memory cell structure makes use of the extraordinary Halleffect in ferromagnetic materials. A memory cell has a ferromagneticlayer possessing perpendicular magnetic anisotropy with magnetic momentoriented perpendicular to the plane of the ferromagnetic layer. Theextraordinary Hall resistance is exhibited between first and second endsof the ferromagnetic layer across a path which intersects a bias currentpath between third and fourth ends of the ferromagnetic structure. Sucha magnetic layer has two stable magnetic orientations: up and down, andgives rise to two different extraordinary Hall values +RH and −RHassociated with bits “0” and “1”. A magnetic memory unit of this typewith enhanced values of RH has been disclosed in U.S. Pat. No. 7,463,447B2 (2008) to A. Gerber, whose disclosure is incorporated herein byreference.

Another memory cell structure makes use of the normal, or ordinary, Halleffect in low carrier density materials such as semiconductors. A memorycell has a low carrier density material film in the shape of a crosswhich constitutes a Hall sensor, on top of which a spacer is placed toprevent electric current leakage, and on top of that a ferromagneticdot, or ferromagnetic dots are deposited. Such magnetic dots have twostable magnetic orientations up and down. Ferromagnetic dots are placedat locations that induce strong magnetic stray flux through the sensor.The normal Hall resistance in the low carrier density film is sensitiveto cumulative stray flux from the magnetized ferromagnetic dots, andgives rise to two different normal Hall values +RH and −RH associatedwith bits “0” and “1”. A magnetic memory unit of this type has beendisclosed in U.S. Patent Application Publication 2008/0205129 to J.Stephenson, B. Shipley, and D. Carothers, whose disclosure isincorporated herein by reference.

In an effort to further increase the ultimate storage density of MRAMs,several multi-state structures and storage schemes have been proposedusing both in-plane and perpendicular anisotropy materials. Angulardependent four-state tunneling magnetoresistance cells were proposed byUemura et al., in “Four-State Magnetoresistance in Epitaxial CoFe-BasedMagnetic Tunnel Junctions,” IEEE Transactions on Magnetics, volume 43,pages 2791-2793 (2007), which is incorporated herein by reference.

Four-state dual spin valve GMR storage was proposed by Law et al., in“Magnetoresistance and Switching Properties of Co—Fe/Pd-BasedPerpendicular Anisotropy Single- and Dual-Spin Valves,” IEEETransactions on Magnetics, volume 44, pages 2612-2615 (2008), which isincorporated herein by reference. A four-state single-layer Fe filmdevice was proposed by Yoo et al., in “Four Discrete Hall ResistanceStates in Single-Layer Fe Film for Quaternary Memory Devices,” AppliedPhysics Letters, volume 95, 202505 (2009), which is incorporated hereinby reference.

A memory cell having two separated ferromagnetic layers was disclosed inU.S. Pat. No. 7,379,321 to D. Ravelosona and B. D. Terris, whosedisclosure is incorporated herein by reference. U.S. Pat. No. 5,361,226,to Taguchi, et al., whose disclosure is incorporated herein byreference, describes a magnetic thin film memory device havinginformation recorded in a magnetic thin film by the direction ofmagnetization. The disclosure states that the film is adapted toreproduce the recorded information on the basis of the voltage generatedas a result of the change of the magnetization direction due to theextraordinary Hall effect.

U.S. Pat. No. 6,727,537, to Wunderlich, whose disclosure is incorporatedherein by reference, describes a magnetic memory device based on easydomain wall propagation and the extraordinary Hall effect, and which isstated to include a “perpendicular-to-plane” magnetic electricallyconductive element.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesa device for storing data. The device includes at least first and secondferromagnetic films and a sensing circuit. The ferromagnetic films bothhave perpendicular magnetic anisotropy that is configured responsivelyto the stored data, and are connected so that an electrical currenttraverses the first and second ferromagnetic films and generatesrespective first and second extraordinary Hall voltages therein. Thesensing circuit is configured to read out the stored data by measuringthe first and second extraordinary Hall voltages.

In some embodiments, each of the first and second ferromagnetic filmsincludes at least one ferromagnetic layer. In an embodiment, eachferromagnetic layer defines one of two memory states. In an exampleembodiment, the first film includes n1 layers, the second film includesn2 layers, wherein n1, n2 are integers greater than 0, and the number ofmemory states is 2^(n1+n2).

In some embodiments, the device includes a current source that isconfigured to generate the electrical current and provide the electricalcurrent to the ferromagnetic films. In an embodiment, the deviceincludes a conductor that connects the ferromagnetic films in series toone another, and the current source is configured to apply theelectrical current so as to traverse the ferromagnetic films and theconductor. In an alternative embodiment, the device includes a conductorthat connects the ferromagnetic films in parallel to one another, thecurrent source is configured to generate and apply a first electricalcurrent to the first ferromagnetic film and a second electrical currentto the second ferromagnetic film, and the sensing circuit is configuredto measure a sum of the first and second extraordinary Hall voltages.

In another embodiment, the sensing circuit is configured to apply areverse magnetic field reciprocity (RMFR) theorem to the films so as tomeasure the first and second extraordinary Hall voltages. In a disclosedembodiment, the device includes a magnetic field generator that isconfigured to store the data in the ferromagnetic films by applying amagnetic field that writes into the ferromagnetic films respectivemagnetic states that represent the data.

In a disclosed embodiment, the magnetic field generator is configured toaccept the data for storage, to produce, responsively to the data, asequence of one or more magnetic field pulses that write the magneticstates, and to apply the sequence to the ferromagnetic films. In anembodiment, the magnetic field generator is configured to produce thesequence responsively to the data and to respective switching magneticfields of the ferromagnetic films. In an embodiment, the magnetic fieldgenerator is configured to produce the magnetic field pulses so as toalternate in polarity and decrease in magnitude along the sequence.

In some embodiments, the first and second ferromagnetic films lie in acommon two-dimensional plane. In alternative embodiments, the first andsecond ferromagnetic films are stacked on top of one another to form athree-dimensional structure. In an embodiment, the first and secondferromagnetic films are characterized by respective, different switchingmagnetic fields.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for data storage. The method includesproviding at least first and second ferromagnetic films, which areconnected to one another and both have perpendicular magneticanisotropy. Data is stored in the first and second ferromagnetic filmsby configuring the perpendicular magnetic anisotropy of the filmsresponsively to the data, so that an electrical current traverses thefirst and second ferromagnetic films and generates respective first andsecond extraordinary Hall voltages therein. The stored data is read outby measuring the first and second extraordinary Hall voltages.

There is also provided, in accordance with an embodiment of the presentinvention, apparatus including first and second hall sensors, aconductor and a processor. The first Hall sensor has a first terminal, asecond terminal, a third terminal and a fourth terminal. The second Hallsensor has a fifth terminal, a sixth terminal, a seventh terminal and aneighth terminal. The conductor connects the third terminal to the fifthterminal. The processor is configured to measure a first potentialbetween the fourth terminal and the sixth terminal while transferring afirst current from the first terminal to the seventh terminal via theconductor, to measure a second potential between the first terminal andthe seventh terminal while transferring a second current from the fourthterminal to the sixth terminal via the conductor, and to determine aresultant voltage generated by the first and second Hall sensors inresponse to the first and second potentials.

In some embodiments, at least one of the first and second Hall sensorsgenerates normal Hall voltages. In some embodiments, least one of thefirst and second Hall sensors generates extraordinary Hall voltages.

There is further provided, in accordance with an embodiment of thepresent invention, a method including providing a first Hall sensorhaving a first terminal, a second terminal, a third terminal and afourth terminal, and a second Hall sensor having a fifth terminal, asixth terminal, a seventh terminal and an eighth terminal. The thirdterminal is connected to the fifth terminal by a conductor. Whiletransferring a first current from the first terminal to the seventhterminal via the conductor, a first potential is measured between thefourth terminal and the sixth terminal. While transferring a secondcurrent from the fourth terminal to the sixth terminal via theconductor, a second potential is measured between the first terminal andthe seventh terminal. A resultant voltage generated by the first andsecond Hall sensors is determined in response to the first and secondpotentials.

There is additionally provided, in accordance with an embodiment of thepresent invention, a memory cell for storing data. The memory deviceincludes at least first and second ferromagnetic films, which both haveperpendicular magnetic anisotropy that is configured responsively to thestored data, and which are connected so that an electrical currenttraverses the first and second ferromagnetic films and generatesrespective first and second extraordinary Hall voltages therein, so asto enable readout of the data from the memory cells by measuring thefirst and second extraordinary Hall voltages.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic presentation of a memory cell including twoferromagnetic elements F1 and F2, a conducting connection between twoelements O—O′ carrying the bias current and electric contacts to twofilms: A, B, F to film F1 and electric contacts C, D, E to film F2,according to an embodiment of the present invention.

FIG. 2 is a schematic presentation of the extraordinary Hall effectvoltage developed by two uniform cells F1 and F2 as a function ofapplied field H, according to an embodiment of the present invention.Both cells exhibit perpendicular magnetic anisotropy with a single stepreversal at switching fields Hs1 and Hs2 respectively. The extraordinaryHall effect voltages exhibited by the films in magnetically saturatedstate are VEHE1 and VEHE2 respectively.

FIG. 3 is a schematic presentation of voltage developed between points F(on film F1) and point E (on film F2) with electric current flowingbetween point A (on film F1) and point D (on film F2) as a function ofapplied magnetic field, according to an embodiment of the presentinvention. Four different voltages can be exhibited by the cell afterreturning to zero field, depending on magnetization orientation in bothcells. Each voltage level corresponds to a different memory state.

FIG. 4a is a schematic presentation of the extraordinary Hall effectsignal developed between point F (on film F1) and point E (on film F2)after subtraction of an offset voltage, according to an embodiment ofthe present invention. The offset voltage is cancelled by subtractingthe results of two measurements: VAD,FE and VFE,AD where VAD,FEindicates the voltage measured between points F and E with currentpassing between A and D, and VFE,AD is the voltage measured betweenpoints A and D with current passing between F and E.

FIG. 4b is a schematic presentation of the extraordinary Hall effectvoltage developed between point F (on film F1) and point C (on film F2)after subtraction of the offset voltage, according to an embodiment ofthe present invention.

FIG. 5a is a schematic presentation of a major hysteresis loop of amagnetic film composed of three layers, each of them exhibiting aperpendicular magnetic anisotropy with different switching fields andextraordinary Hall resistances, according to an embodiment of thepresent invention.

FIG. 5b is a schematic presentation of minor hysteresis loops exhibiting8 distinct stable voltages at zero field, according to an embodiment ofthe present invention.

FIG. 6a shows a combination of two films: one with a single layer andsingle step magnetization reversal and the second with two layers anddouble-step reversal, according to an embodiment of the presentinvention.

FIG. 6b is a schematic presentation of EHE voltage VEHE,FE measuredbetween points F (film 1) and E (film 2) at different minor loops and ata major loop after removal of a longitudinal voltage Vl, according to anembodiment of the present invention.

FIG. 6c is a schematic presentation of EHE voltage VEHE,FC measuredbetween points F (film 1) and C (film 2) at different minor loops and ata major loop after removal of the longitudinal voltage Vl, according toan embodiment of the present invention.

FIG. 7 is a schematic presentation of a combination of two double-layer,double-step reversal films exhibiting sixteen different voltage statesat zero field after application of different sequences of magnetic fieldpulses, according to an embodiment of the present invention.

FIG. 8 is a schematic presentation of a combination of two triple-layer,triple-step reversal films exhibiting sixty four different voltagestates at zero field after application of different sequences ofmagnetic field pulses, according to an embodiment of the presentinvention.

FIG. 9 shows an experimental realization of a memory cell composed oftwo single-layer single-step reversal films exhibiting four differentEHE voltage states at zero field, according to an embodiment of thepresent invention.

FIG. 10 shows an experimental realization of a memory cell composed of asingle-layer single-step reversal film and a double-layer double-stepreversal film exhibiting eight different EHE voltage states at zerofield, according to an embodiment of the present invention.

FIG. 11 shows an experimental realization of a memory cell composed oftwo double-layer double-step reversal films exhibiting sixteen differentEHE voltage states at zero field, according to an embodiment of thepresent invention.

FIGS. 12 and 13 are schematic views of alternative memory cells,according to embodiments of the present invention.

FIG. 14 is a block diagram that schematically illustrates a memorydevice, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

A memory cell according to an embodiment of the present inventioncomprises two ferromagnetic films, typically in the form of layers,exhibiting perpendicular magnetic anisotropy. In the absence of anexternal magnetic field, the magnetic moments of both films are orientedperpendicular to the plane of the layer and can exhibit either a singlemagnetization value or a plurality of magnetization values correspondingto the relative magnetization orientations of intra-film layers.

Each film may be characterized by either a single value of the switchingfield or by a plurality of switching fields corresponding to a pluralityof magnetization sets. Consequently, reversal of magnetic moment from agiven to an opposite direction takes place by either a single step or bya plurality of well defined steps. Each ferromagnetic film may exhibiteither a single absolute value of the extraordinary Hall resistance or aplurality of the extraordinary Hall resistances corresponding to the setof magnetization states of a given film. (The extraordinary Hall effectis also sometimes referred to as the anomalous Hall effect.)

In a disclosed embodiment a bias current circuit is adapted to produce abias current having a bias path between the first and the second film ofthe cell, so that the same bias current flows in the first and secondfilms. A voltage sensing circuit is adapted to measure a voltage levelbetween the two films of the cell, the level varying in accordance witheach one of a plurality of predetermined extraordinary Hall resistancescorresponding to a plurality of magnetic states of the two-film system.

The number of discrete memory states N demonstrated by a pair of filmsdepends on the number of stable magnetic states exhibited by the films.For two films exhibiting single step magnetization reversal anddifferent values of switching fields and extraordinary Hall resistancesthe number of memory states is 4. For two films exhibiting two-stepmagnetization reversals and different values of respective switchingfields and extraordinary Hall resistances the number of memory states is16. For two films exhibiting three-step magnetization reversals anddifferent values of respective switching fields and extraordinary Hallresistances the number of memory states is 64. In general, for a systemcomposed of two films the number of possible memory states is 2^((n) ¹^(+n) ² ⁾, where n₁ and n₂ are the number of distinct sections or layersexhibiting different switching fields and extraordinary Hall resistancesin the first and the second films respectively.

For improved resolution of the memory reading process a signal detectionprotocol employs two four probe measurements. For two pads A and Battached to the first film of the cell and pads C and D attached to thesecond film of the cell, two voltage measurements are taken: V_(AD,BC)and V_(BC,AD) where V_(AD,BC) indicates the value of voltage developedbetween contacts B of the first film and C of the second film with biascurrent being passed between contact A of the first film and contact Dof the second film. V_(BC,AD) indicates the value of voltage developedbetween contacts A of the first film and D of the second film when biascurrent is passed between contact B of the first film and contact C ofthe second film. By addition or subtraction of V_(AD,BC) and V_(BC,AD)an odd in-field extraordinary Hall effect component is extracted fromthe total measured inter-film voltage.

Memory cells in accordance with embodiments of the present inventionmake use of the extraordinary Hall effect for increased data storagecapacity. FIG. 1 presents a schematic view of a memory cell, accordingto an embodiment of the present invention. A typical memory cellcomprises two ferromagnetic films F1 and F2, each in the form of a planelayer, and each of the films exhibiting perpendicular magneticanisotropy. In the absence of an external magnetic field magneticmoments of both films are oriented perpendicular to the plane of thelayer. The magnetic moments may have either a single magnetization valueor a plurality of magnetization values corresponding to relativemagnetization orientations of intra-film sections, the sectionscomprising multiple different ferromagnetic layers. Each film ischaracterized by either a single value of the switching field or by aplurality of switching fields, therefore reversal of magnetic momentfrom a given to an opposite direction takes place by either a singlestep or by a plurality of well defined steps. Each ferromagnetic filmexhibits either a single absolute value of the extraordinary Hallresistance or a plurality of the extraordinary Hall resistancescorresponding to the set of magnetization states of that film.

In the memory cell, films F1 and F2 are connected in series, and a biascurrent source (not shown in the figure) produces a bias current from Ato D, including a bias path OO′ between the first and the second film ofthe cell, so that there is a common current in films F1 and F2. Avoltage sensing circuit (also not shown) is adapted to measure a voltagelevel between two films of the cell. As described below the voltagelevel may be used to determine a plurality of predeterminedextraordinary Hall resistances corresponding to a plurality of magneticstates of the two-film system.

Each of the two films comprising the memory cell has a ferromagneticstructure which includes at least one layer, or a plurality of layers,exhibiting perpendicular magnetic anisotropy. The ferromagnetic layershave magnetic moments, lying perpendicular to the planes of the layers,which are set in accordance with one orientation or a plurality ofmagnetic orientation sets of the ferromagnetic structure. Theferromagnetic structure exhibits one of a plurality of predeterminedextraordinary Hall resistances in accordance with the magneticorientation set.

In the present disclosure, a ferromagnetic layer is defined by itsswitching field, i.e., the magnetic field required to switch themagnetic orientation of the layer, and by the extraordinary Hall effect(EHE) resistance of the layer. As is explained further below in relationto the examples of FIGS. 9-11, a given ferromagnetic layer may comprisea number of strata, including ferromagnetic and non-ferromagneticstrata, which together have one or more switching fields and one or moreEHE resistances. Furthermore, as is also exemplified in the cases ofFIGS. 9-11, the switching fields and the EHE resistances may be setaccording to the physical dimensions of the ferromagnetic andnon-ferromagnetic strata, the proximity of the strata to a substrate orseed layer, and the dimensions and type of the substrate or seed layer.

The extraordinary Hall resistance is exhibited between first and secondends of the ferromagnetic structure (points B and F in film F1 andpoints C and E in film F2) across a path that intersects a bias currentpath between third and fourth ends of the ferromagnetic films (points Aand O in film F1 and points O′ and D in film F2).

In uniform ferromagnetic films the Hall effect voltage VH can beexpressed as

$\begin{matrix}{V_{H} = {\frac{I}{t}\left( {{R_{0}B} + {\mu_{0}R_{E\; H\; E}M}} \right)}} & \lbrack 1\rbrack\end{matrix}$

where I is electrical current, t the sample thickness, R0 and REHE arethe ordinary and extraordinary Hall effect (EHE) coefficients, and B andM are magnetic field induction and magnetization components normal tothe film respectively.

The extraordinary Hall effect, represented by coefficient REHE inferromagnetic structures, is generally attributed to scattering ofelectrons in the presence of spin-orbit interactions. In mostferromagnets the EHE component dominates over the ordinary Hallcomponent, and VH is proportional to the film magnetization (theordinary Hall component will be neglected in the rest of the descriptionunless stated otherwise). Given a ferromagnetic film with aperpendicular magnetic anisotropy exhibiting hysteresis in itsmagnetization response to applied magnetic field, the remanentout-of-plane magnetization (magnetization exhibited by the materialafter reducing the applied field to zero) can be detected by measuring atransverse voltage generated due to the EHE, thus reading the magneticstate of the film.

FIG. 2 is a schematic presentation of the extraordinary Hall voltagedeveloped in film F1 between points B and F and in film F2 betweenpoints C and E measured as a function of magnetic field appliedperpendicular to the planes of the films with bias current appliedbetween points A and D, according to an embodiment of the presentinvention.

Each film has a uniform structure and exhibits a correlated, uniform,single step magnetization reversal between the up- and down-magnetizedstates. The hysteresis loop of film F1 is characterized by coercive(switching) field Hs1 and the magnitude of the Hall voltage in thesaturated state VH1 in a positive field and (−VH1) in a negative field.Hysteresis loop of film F2 is characterized by coercive (switching)field Hs2 and the magnitude of the Hall voltage in the saturated stateVH2. After saturating magnetization in high positive or negative fieldsand reducing the applied field to zero, the film F1 exhibits remanentHall voltage +VH1 or −VH1, while film F2 exhibits remanent Hall voltage+VH2 or −VH2.

A magnetic state of the memory cell is determined according to thevoltage developed between two films of the cell. The magnetic state ofthe device is read by measuring voltage VAD,FC which is defined as thevoltage between points F and C with bias current passing betweencontacts A and D.

VAD,FC has two contributions: the voltage across the longitudinalresistance of the electrical connection between films F1 and F2, denotedas VL, and a second contribution comprising half the sum of the Hallvoltages VAD,FB and VAD,EC developed in films F1 and F2 respectively(VAD,FB is the Hall voltage VH1 exhibited between contacts FB with biascurrent flowing between contacts A and D; VAD,EC is the Hall voltage VH2exhibited between contacts EC with bias current flowing between contactsA and D:

$\begin{matrix}{V_{{A\; D},{F\; C}} = {{V_{L} + \frac{V_{{A\; D},{F\; B}} + V_{{A\; D},{E\; C}}}{2}} = {{V_{L} + \frac{V_{H\; 1} + V_{H\; 2}}{2}} = {V_{L} + {\frac{\mu_{0}I}{2}\left\lbrack {\frac{R_{{E\; H\;{E1}}\;}M_{1}}{t_{1}} + \frac{R_{E\; H\; E\; 2}M_{2}}{t_{2}}} \right\rbrack}}}}} & \lbrack 2\rbrack\end{matrix}$

Eq. 2 is obtained by integration of the electric field along the pathfrom point F to C, as shown schematically in FIG. 1. Segments a and care orthogonal to the current flow, and contribute the Hall voltages,and segment b is parallel to the current and contributes thelongitudinal resistance related voltage VL. Thus, the voltage dropdeveloped between two films of the magnetic memory cell depends on theextraordinary Hall resistance developed in each of the films.

Alternatively, the magnetic state of the device is read by measuringvoltage VAD,FE which is defined as the voltage between points F and Ewith bias current passing between contacts A and D. In this case VAD,FEis given by:

$\begin{matrix}{V_{{A\; D},{F\; E}} = {{V_{L} + \frac{V_{{A\; D},{F\; B}} - V_{{A\; D},{E\; C}}}{2}} = {{V_{L} + \frac{V_{H\; 1} - V_{H\; 2}}{2}} = {V_{L} + {\frac{\mu_{0}I}{2}\left\lbrack {\frac{R_{{E\; H\;{E1}}\;}M_{1}}{t_{1}} - \frac{R_{E\; H\; E\; 2}M_{2}}{t_{2}}} \right\rbrack}}}}} & \lbrack 3\rbrack\end{matrix}$

FIG. 3 is a schematic presentation of voltage developed between point F(on film F1) and point E (on film F2) with electric current flowingbetween point A (on film F1) and point D (on film F2) as a function ofapplied magnetic field, according to an embodiment of the presentinvention. Four different voltages can be exhibited by the cell afterreturning to zero field, depending on the magnetization orientation inboth cells. Each voltage level corresponds to a different memory state.

Voltages V_(AD,FC) in Eq. 2 and V_(AD,FE) in Eq. 3 contain the combinedEHE term and the longitudinal resistance term V_(L). In regularferromagnetic materials like Co, Fe, Ni and their alloys theextraordinary Hall resistivity is about two orders of magnitude smallerthan the longitudinal resistivity. Therefore, the longitudinal voltageV_(L) may be much larger than the variable EHE term, resulting in asmall relative change of the output signal when switching from onememory state to another.

In some embodiments of the present invention, suppression of thebackground longitudinal voltage may be achieved by application of areverse magnetic field reciprocity (RMFR) theorem, which is well knownin the art. According to the RMFR theorem:V _(ab,cd)(H)=V _(cd,ab)(−H)  [4]Where a, b, c, and d are four arbitrary locations in a system. Inferromagnetic materials magnetization replaces the applied magneticfield, giving in our case:V _(AD,FC)(M)=V _(FC,AD)(−M)  [5]

The longitudinal resistance voltage V_(L) is an even function of themagnetization: V₁(M)=V₁(−M), i.e. its value does not change when themagnetization reverses under application of a magnetic field withreversed polarity. The EHE voltage is an odd function of magnetization,i.e.:V _(AD,FB)(M)=−V _(AD,FB)(−M)  [6]By performing two measurements V_(AD,FC)(H) and V_(FC,AD)(H) at the sameapplied magnetic field or in the same magnetization state and bysubtracting one signal from another one gets a sum of Hall effectvoltages exhibited by both films free from the background componentV_(L):V _(AD,FC)(M)−V _(FC,AD)(M)=V _(AD,FC)({right arrow over (M)})−V_(AD,FC)(−{right arrow over (M)})=V _(AD,FB)({right arrow over (M)})+V_(AD,EC)( M )=V _(EHE1) +V _(EHE2)  [7]For measurements performed using contacts A, D, F, E the result ofsubtraction will be:V _(AD,FE)(M)−V _(FE,AD)(M)=V _(EHE1) −V _(EHE2)  [8]

The EHE components obtained after reduction of the longitudinal voltagebetween points FE V_(EHE,FE) and FC V_(EHE,FC) are shown schematicallyin FIGS. 4a and 4 b.

Eqs. 2, 3 were developed for two films, each possessing a single valueHall resistance in the saturated state and a single well definedswitching field. It is known in the art that one can fabricate magneticunits demonstrating hysteresis loops containing a plurality ofintermediate magnetization levels exhibiting switching at differentcorresponding switching fields. A film containing a plurality ofparallel layers exhibiting a plurality of respective switching fieldsand a plurality of the respective EHE resistances will generate a totalEHE voltage of:

$\begin{matrix}{V_{EHE} = {\frac{\mu_{0}}{2}{\sum\limits_{i}\frac{I_{i}R_{{EHE},i}M_{i}}{t_{i}}}}} & \lbrack 9\rbrack\end{matrix}$

where I_(i) is electric current flowing along the layer i, R_(EHE,i),M_(i), t_(i) are the EHE coefficient, the magnetization normal to thelayer plane and the thickness of the respective section i. Thus, a filmcontaining two parallel sections with the respective values of I₁,R_(EHE,1), M₁ and t₁ will exhibit four remanent EHE voltagescorresponding to states M₁M₂, M₁(−M₂), (−M₁)M₂ and (−M₁)(−M₂) givenrespectively by a set:

$\begin{matrix}{V_{EHE} = {\frac{\mu_{0}}{2}\left( {\frac{I_{1}R_{{EHE},1}M_{1}}{t_{1\;}} + \frac{I_{2}R_{{EHE},2}M_{2}}{t_{2}}} \right)}} & \left\lbrack {10a} \right\rbrack \\{V_{EHE} = {\frac{\mu_{0}}{2}\left( {\frac{I_{1}R_{{EHE},1}M_{1}}{t_{1}} - \frac{I_{2}R_{{EHE},2}M_{2}}{t_{2}}} \right)}} & \left\lbrack {10b} \right\rbrack \\{V_{EHE} = {\frac{\mu_{0}}{2}\left( {{- \frac{I_{1}R_{{EHE},1}M_{1}}{t_{1}}} + \frac{I_{2}R_{{EHE},2}M_{2}}{t_{2\;}}} \right)}} & \left\lbrack {10c} \right\rbrack \\{V_{EHE} = {{- \frac{\mu_{0}}{2}}\left( {\frac{I_{1}R_{{EHE},1}M_{1}}{t_{1\;}} + \frac{I_{2}R_{{EHE},2}M_{2}}{t_{2}}} \right)}} & \left\lbrack {10d} \right\rbrack\end{matrix}$

A film composed of three parallel layers will exhibit eight differentHall voltage signals corresponding to eight different combinations ofremanent magnetizations in its three layers. In general, magnetic filmcontaining n different layers can exhibit 2^(n) different EHE signals atzero field.

The writing procedure of different memory states in a single multilayerfilm is presented schematically in FIGS. 5a and 5b for an example of afilm composed of three layers, each of them exhibiting a perpendicularmagnetic anisotropy with different switching fields and extraordinaryHall resistances.

FIG. 5a presents a major hysteresis loop in which the EHE voltage ismeasured as a function of applied magnetic field swept between highpositive field to high negative field and back to high positive field,according to an embodiment of the present invention. Magnetizationreversal from −M state towards +M state proceeds in three distinct stepsat three switching fields H_(s1), H_(s2) and H_(s3) during the fieldsweep from negative field towards a high positive field, andmagnetization reversal from +M state towards −M state proceeds in threedistinct steps at switching fields −H_(s1), −H_(s2) and −H_(s3) duringthe field sweep from positive field towards a high negative field. FIG.5b presents a set of minor loops with a total of eight differentremanent states at zero field, according to an embodiment of the presentinvention.

In an embodiment, a protocol of writing different memory states can bedefined as follows. By way of example, the protocol described hereinassumes starting by resetting the cell using a high value magnetic fieldpulse, Those having ordinary skill in the art will be able to adapt thedescription below, to write different memory states, to generatedifferent protocols, including protocols with fewer pulses to achieve agiven memory state.

Define the magnetic writing fields H_(w1), H_(w2), and H_(w3), producedby a magnetic field generator, such that H_(S2)>H_(w1)>H_(S1),H_(S3)>H_(w2)>H_(S2), and H_(w3)>H_(S3).

State 1 is achieved by applying field H_(w3) and returning to zero.

State 2 is achieved by applying the following sequence of field pulses:H_(w3), −H_(W1) and returning to zero.

State 3 is achieved by applying the following sequence of field pulses:H_(w3), −H_(W2), H_(w1) and returning to zero.

State 4 is achieved by applying the following sequence of field pulses:H_(w3), −H_(W2) and returning to zero.

State 5 is achieved by applying the following sequence of field pulses:−H_(w3), H_(W2), and returning to zero.

State 6 is achieved by applying the following sequence of field pulses:−H_(w3), H_(W2), −H_(w1) and returning to zero.

State 7 is achieved by applying the following sequence of field pulses:−H_(w3), H_(W1) and returning to zero.

State 8 is achieved by applying field −H_(w3) and returning to zero.

A memory cell disclosed herein contains two electrically connectedfilms, wherein each of the films contains one or a plurality of parallellayers, each of the layers having a respective thickness, EHEcoefficient and magnetization.

The voltage developed between two electrically connected magnetic filmsdescribed above, is given by:

$\begin{matrix}{V_{{AD},{FC}} = {{V_{L} + \frac{V_{{AD},{FB}} + V_{{AD},{EC}}}{2}} = {V_{L} + {\frac{\mu_{0}}{2}{\sum\limits_{i}\frac{I_{i}R_{{EHE},i}M_{i}}{t_{i}}}}}}} & \lbrack 11\rbrack\end{matrix}$

where V_(L) is the voltage from the longitudinal resistance contributedby a current carrying portion of film 1, a current carrying portion offilm 2 and the electrical connection between film 1 and film 2. Index iincludes all parallel sections of film 1 and film 2 together. For thefirst film containing n₁ layers and the second film containing n₂ layersthe number of different combinations of magnetization and, therefore anumber of different output voltage signals is 2^(n) ¹ ^(+n) ² . Sinceeach voltage signal corresponds to a respective memory state, the numberof memory states per cell is 2^(n) ¹ ^(+n) ² . A voltage differenceamong different memory states is: ΔV_(i,j)=V_(AD,FC) ^(i)−V_(AD,FC)^(j),

where V_(AD,FC) ^(i) and V_(AD,FC) ^(j) correspond to differentmagnetization states.

FIGS. 6a, 6b, and 6c are schematic presentations of EHE voltagesdeveloped in a cell containing a first film composed of a single layerand exhibiting a single step magnetization reversal and a second filmcomposed of two layers and exhibiting a double-step magnetizationreversal, according to an embodiment of the present invention.

FIG. 6a shows the EHE voltage developed at contacts BF of the first filmexhibiting a single step reversal at switching fields ±H_(s1) and atcontacts CE of film 2 exhibiting a double step reversal at fields±H_(s2) and ±H_(s3).

FIG. 6b presents the EHE voltage V_(EHE,FE) measured between points F(film 1) and E (film 2) after removal of longitudinal voltage V₁.V_(EHE,FE) equals half the difference between the EHE signal across film1 (V_(AD,BF)) and the EHE signal across film 2 (V_(AD,CE)) (see Eq. 3).

FIG. 6c presents the EHE voltage V_(EHE,FC) measured between points F(film 1) and C (film 2) after removal of longitudinal voltage V₁.V_(EHE,FC) equals half the sum of the EHE signal across film 1(V_(AD,BF)) and the EHE signal across film 2 (V_(AD,CE)) (see Eq. 2).

The eight memory states available in this memory cell may be written inthe following pulse sequences:

Define the writing fields H_(w1), H_(w2), and H_(w3), such thatH_(S2)>H_(w1)>H_(S1), H_(S3)>H_(w2)>H_(S2), and H_(w3)>H_(S3).

State 1 is achieved by applying field H_(w3) and returning to zero.

State 2 is achieved by applying field sequence H_(w3), −H_(W1) andreturning to zero.

State 3 is achieved by applying field sequence H_(w3), −H_(W2), H_(w1)and returning to zero.

State 4 is achieved by applying field sequence H_(w3), −H_(W2) andreturning to zero.

State 5 is achieved by applying field sequence −H_(w3), H_(W2) andreturning to zero.

State 6 is achieved by applying field sequence −H_(w3), H_(W2), −H_(w1)and returning to zero.

State 7 is achieved by applying field sequence −H_(w3), H_(W1) andreturning to zero.

State 8 is achieved by applying field −H_(w3) and returning to zero.

In the examples described herein, the magnetic states are written byapplying a sequence of pulses that alternate in polarity and decrease inmagnitude along the sequence. This choice, however, is made purely byway of example. In alternative embodiments, the magnetic states can bewritten using any other suitable sequence of magnetic pulses, or in anyother suitable way.

In an alternative embodiment of a memory cell the switching fields offilms F1 and F2 may be ordered in different ways. For example, in a cellwith film F1 exhibiting a double-step magnetization reversal atswitching fields H_(s1) and H_(s2), and film F2 exhibiting a single-stepmagnetization reversal at field H_(s3), H_(s3) can be larger or smallerthan H_(s2) and larger or smaller than H_(s1).

In a further alternative embodiment of a memory cell the first film iscomposed of two layers and exhibits a double-step magnetization reversaland the second film is composed of two layers and exhibits a double-stepmagnetization reversal, with all component layers of two filmsexhibiting different values of switching fields and EHE resistances. Inthis case the number of available magnetic memory states is 16, asillustrated in FIG. 7.

In a yet further alternative embodiment of a memory cell the first filmis composed of three layers and exhibits a triple-step magnetizationreversal and the second film is composed of three layers and exhibits atriple-step magnetization reversal, with all component layers of twofilms exhibiting different values of switching fields and EHEresistances. In this case the number of available magnetic memory statesis 64, as illustrated in FIG. 8.

The ferromagnetic structure of all films of a memory cell may be madefrom a single layer of material exhibiting perpendicular magneticanisotropy or multiple layers of materials exhibiting perpendicularmagnetic anisotropy. Typically, in order to reduce an inter-layercoupling and conserve different magnetic properties in different layers,the layers are separated by non-magnetic spacer layer. Ferromagneticlayers may be made from a variety of materials known in the art,including, but not limited to, Co, Co/Pd multilayers, Co/Pt multilayers,Fe/Pt multilayers, Fe/Pd multilayers, CoPt, CoPd, FePt, FeTb alloys,diluted magnetic semiconductors, and half metals, all possessingperpendicular magnetic anisotropy. The spacer layer may be made fromnon-magnetic electrically conductive material, such as Pd, Pt, Cu, Ru,Ti or other material. Alternatively, the spacer layer may be made fromelectrically insulating material such as SiO₂, alumina (Al₂O₃), or MgO.Further alternatively, the spacer layer may be made from non-magneticconductive material containing high concentration of insulatingimpurities, such as PdSiO₂, CuSiO₂, etc. Insulating impurities can beadded to a metallic spacer in order to increase electrical resistance ofthe spacer layer and thus to reduce the fraction of electric currentflowing along the spacer layer as compared with current flowing alongferromagnetic layers. Alternatively, films with multiple layers can bemade with no spacer layer, such as thin Co/Pd multilayers grown on athin Pd seed under-layer.

To achieve ferromagnetic layers with different switching fields anddifferent EHE resistances different ferromagnetic materials, differentseed layers, different thicknesses and different polarities of theextraordinary Hall effect (EHE coefficient R_(EHE) can be positive ornegative in different materials) may be utilized as known in the art.

Magnetic memory cells disclosed herein are visualized to be part of anetwork making a portion of, or an entire, magnetic random access memorydevice or other memory device. It will be understood that the physicallocation and relative positioning of films comprising a given cell isnot restricted. For example, a magnetic memory cell containing twoferromagnetic films exhibiting different switching fields and differentEHE resistances may be located in proximity to each other in the sameplane of a magnetic memory device. In an alternative embodiment, twofilms composing a given cell may be located in different layers orplanes of a magnetic memory device. Two different fabrication methodsmay be implemented for these two cases. For films located in the sameplane of the device the selective fabrication of one type of films andthe selective fabrication of the second type of films are performed atthe same device layer. For the alternative embodiment, identical filmsof the first type may be fabricated in the first layer of the device,identical films of the second type may be fabricated at a differentlayer of the device, and electrical interconnections between the filmsmay be fabricated vertically between the layers of the device.

It will be understood that films comprising a memory cell are spatiallyseparated. Typically, the spatial separation may be adjusted to improvethe temporal stability of a cell, so that, for example, a stronger layerdoes not adversely affect a weaker layer. In addition the spatialseparation may be adjusted to facilitate the writing procedure, forexample allowing high field pulses to be applied to stronger layers, andlow field pulses to be applied to weaker layers. The writing proceduremay be implemented using a field generating network, such as a grid ofcurrent-carrying wires, and the procedure may be optimized by adjustingboth the film spatial separation and the arrangement of the network.

To illustrate the feasibility of magnetic memories disclosed herein wepresent in FIG. 9 an experimental implementation of a four-states cell.FIG. 9 presents the EHE voltage measured between contacts F and E of twosingle layer films, according to an embodiment of the present invention.The first and second films were fabricated by e-beam deposition of Co/Pdmulti-strata structures composed of 2 Å thick Co and 9 Å thick Pd stratarepeated six times [Co 2 Å Pd 9 Å]₆, so that each single layer film is amulti-strata unit. The depositions were on 100 Å thick Pd and 50 Å thickPd seed substrates respectively. In this example, each multi-stratastructure corresponded to a single ferromagnetic layer, each of thestructures having a given switching field and a given EHE resistance. Asis illustrated in the graph each single layer (multi-strata structure)has a different EHE resistance, which is assumed to be due to thedifferent thicknesses of Pd seed substrate.

FIG. 10 presents an experimental implementation of an eight-statesmemory cell, according to an embodiment of the present invention. Thefirst film exhibiting a single step reversal was fabricated by e-beamdeposition of a Co/Pd multi-strata structure composed of 2 Å thick Coand 9 Å thick Pd strata repeated six times [Co 2 Å Pd 9 Å]₆ deposited ona 100 Å thick Pd seed substrate on top of a glass substrate. The secondfilm exhibiting a double step magnetization reversal was fabricated bye-beam deposition of a Co/Pd multi-strata structure composed of 2 Åthick Co and 9 Å thick Pd strata repeated six times [Co 2 Å Pd 9 Å]₆deposited on a 75 Å thick Pd seed substrate on top of a GaAs substrate.The different properties of the multi-strata structures, i.e., the factthat the first film has single step reversal whereas the second film hasdouble step reversal, are assumed to be due to the different substratesupon which the structures are deposited.

FIG. 11 presents an experimental implementation of a sixteen-statesmemory cell, according to an embodiment of the present invention. Thefirst film of the cell exhibiting a double step magnetization reversalwas fabricated by e-beam deposition of a Co/Pd multilayer structurecomposed of 2 Å thick Co and 9 Å thick Pd layers repeated six times [Co2 Å Pd 9 Å]₆ deposited on a 75 Å thick Pd seed layer on top of a GaAssubstrate. (This film has the same form as the second film of theexample of FIG. 10.) The second film of the cell exhibiting a doublestep magnetization reversal was fabricated on a GaAs substrate by e-beamdeposition of a Co/Pd multilayer structure, the first multilayercomponent comprising 2 Å thick Co and 9 Å thick Pd layers repeated sixtimes [Co 2 Å Pd 9 Å]₆, followed by 30 Å thick Pd spacer, followed bythe second multilayer component comprising 4 Å thick Co and 9 Å thick Pdlayers repeated six times [Co 4 Å Pd 9 Å]₆.

FIG. 12 and FIG. 13 are schematic views of alternative memory cells,according to embodiments of the present invention. In the memory cellillustrated in FIG. 1, the films constituting the cell are connected inseries, so that the same bias current (between A and D) traverses filmF1 and film F2. In contrast, in the alternative memory cell illustratedin FIG. 12, films F1 and F2 are connected in parallel. Similarly, in thealternative memory cell illustrated in FIG. 12, films F1, F2, and F3 areconnected in parallel. In both alternative cells there is a biascurrent, between A and C, which is distributed between the films of thealternative cells. Typically the distribution is not equal, in whichcase the bias currents in each cell are different. Alternatively, thedistribution may be equalized or otherwise modified, for example byadjusting the film resistance and/or by adding resistance.

In the case of the cell illustrated in FIG. 12, the number of memorystates per cell is 2^(n) ¹ ^(+n) ² , where film F1 has n₁ ferromagneticlayers and film F2 has n₂ layers, as described above with reference toequation (11). In the case of the cell illustrated in FIG. 13, thenumber of memory states per cell is 2^(n) ¹ ^(+n) ² ^(+n) ³ , where filmF3 has n₃ layers. In the parallel memory cell configuration illustratedin FIG. 12, a conductor EE′ may connect parts of films F1 and F2 wherethe extraordinary Hall voltage is developed, although in someembodiments having two films in parallel, there is no conductor EE′. Inthe parallel memory cell illustrated in FIG. 13, conductor EE′ connectsfilms F1 and F2, and a conductor GG′ connects films F2 and F3. Theconductors in the cell of FIG. 13 also connect parts of the films wherethe extraordinary Hall voltage is developed. Because of the presence ofthe conductors between the films, the memory state of the cell of FIG.12 may be determined by direct measurement of the voltage between pointsB and D of the cell. For a similar reason, the memory state of the cellof FIG. 13 may also be determined by direct measurement of the voltagebetween points B and D of the cell, i.e. by measurement between the“lowest” part of film F1, and the “highest” part of film F3.

In general, for a memory cell having k films connected in parallel (k apositive integer), where the p^(th) film (1≦p≦k) has n_(p) ferromagneticlayers, the cell has 2^(n) ¹ ^(+ . . . n) ^(p) ^(+ . . . +n) ^(k) memorystates. Such a memory cell may have (k−1) conductors connecting thefilms. The different states of the general cell may be found bymeasuring the voltage between the “lowest” part of the first film, andthe “highest” part of the k^(th) film, i.e., the parts of the films thatare not connected to conductors, and where the extraordinary Hallvoltage is developed.

In the case of films connected in parallel, the RMFR system describedabove may be implemented, e.g., to reduce any mismatch that may occur inlocations of the contacts measuring the Hall voltage. For example, inFIG. 12, contact B may not be exactly opposite contact E, and/or E′ maynot be exactly opposite contact D. The RMFR system may be used to allowfor such mis-alignment.

The voltage offset reduction scheme as described above can also beimplemented in other devices made of two normal Hall effect sensors inseries or in parallel, which may not exhibit the extraordinary Halleffect. In a different embodiment corresponding to FIG. 1, films F1 andF2 may each be made of a low carrier density material film in the shapeof a cross which constitutes a Hall sensor, on top of which a spacer isplaced to prevent electric current leakage. On top of the spacer aferromagnet, or ferromagnets, may be deposited, as disclosed in U.S.Patent Application Publication 2008/0205129 to J. Stephenson, B.Shipley, and D. Carothers, whose disclosure is incorporated herein byreference. Such ferromagnets have a set of stable magnetic orientations,and may be placed at locations that induce a strong magnetic stray fluxthrough the sensor. The normal Hall resistance in the low carrierdensity film is sensitive to cumulative stray flux from the magnetizedferromagnets, and gives rise to a set of different Hall voltages. Byconnecting the films as illustrated in FIG. 1, and measuringV_(AD,FE)−V_(FE,AD), or V_(AD,FC)−V_(FC,AD) with, for example, aprocessor, the Hall voltage is separated from the offset voltage by useof the reverse magnetic field reciprocity.

Generally, when multiple memory cells (e.g., of the type shown in FIG.12 or FIG. 13) are arranged in a memory array, each memory cell has asingle external address, typically corresponding to the cell's row andcolumn, and thus has four external terminals. Storage layers within thememory cell are interconnected. The sensing circuit typically does notmeasure the Extraordinary Hall voltages of the various layersseparately, but rather a combination of them, wherein each combinationgenerates a different signal.

The disclosed techniques provide a possibility of fabricating 3-D memorywith a freedom in positioning the elements of the same cell at differentlevels and locations. This design flexibility is important, for example,in mitigating inter-dot coupling and programming of a given memoryelement without affecting the others.

FIG. 14 is a block diagram that schematically illustrates a memorydevice 20, according to an embodiment of the present invention. In thepresent example, memory device 20 comprises one or more memory cells 22that are based on the disclosed techniques, such as the memory cellsshown in FIG. 1, 12 or 13. A current source 24 generates electricalcurrents (also referred to herein as bias currents) that flow in memorycells 22. A magnetic field generator 26 generates magnetic fields, forexample in order to store data in memory cells 22 according to theprotocols explained above. A sensing circuit 28 senses the Hall voltagesof the memory cells for retrieving the data stored in the memory cells.

The configuration of memory device 20 shown in FIG. 14 is an exampleconfiguration, which is chosen purely for the sake of conceptualclarity. In alternative embodiments, any other suitable memory deviceconfiguration can be used.

It will be understood that memory cells according to embodiments of thepresent invention may be constructed in a substantially planar or twodimensional format, where each film of a cell lies on a common plane,Alternatively, memory cells according to embodiments of the presentinvention may be constructed in a substantially three dimensionalformat, for example having films of the cell on different planes, oreven stacked substantially vertically on each other.

In a three-dimensional multi-layer structure (as well as in otherconfigurations), the magnetic field for writing the magnetic states canbe generated separately in different layers by multiple field generationnetworks.

In the embodiments described herein, the magnetic states are written(programmed) by applying magnetic pulses. The disclosed memory cellconfigurations and readout techniques, however, are in no way limited tothis kind of programming. For example, in alternative embodiments themagnetic states can be written by applying current pulses through thememory cell in order to reverse the magnetization polarity, e.g., usingspin torque transfer. The disclosed memory cell configurations andreadout techniques can be used with such current-based programming, orwith any other suitable programming scheme.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art. Documents incorporated by reference inthe present patent application are to be considered an integral part ofthe application except that to the extent any terms are defined in theseincorporated documents in a manner that conflicts with the definitionsmade explicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

The invention claimed is:
 1. Apparatus, comprising: a first Hall effectsensor having a first terminal, a second terminal, a third terminal anda fourth terminal; a second Hall effect sensor having a fifth terminal,a sixth terminal, a seventh terminal and an eighth terminal; a conductorconnecting the third terminal to the fifth terminal; and a processor,which is configured to measure a first potential between the fourthterminal and the sixth terminal while transferring a first bias currentfrom the first terminal to the seventh terminal via the conductor, tomeasure a second potential between the first terminal and the seventhterminal while transferring a second bias current from the fourthterminal to the sixth terminal via the conductor, to determine aresultant voltage generated by the first and second Hall effect sensorsin response to the first and second potentials, and to determine a valuestored in the first and second Hall effect sensors responsive to thedetermined resultant voltage.
 2. The apparatus according to claim 1,wherein at least one of the first and second Hall effect sensorsgenerates normal Hall voltages.
 3. The apparatus according to claim 1,wherein least one of the first and second Hall effect sensors generatesextraordinary Hall voltages.
 4. A method, comprising: providing a firstHall effect sensor having a first terminal, a second terminal, a thirdterminal and a fourth terminal; providing a second Hall effect sensorhaving a fifth terminal, a sixth terminal, a seventh terminal and aneighth terminal; connecting the third terminal to the fifth terminal bya conductor; while transferring a first current from the first terminalto the seventh terminal via the conductor, measuring a first potentialbetween the fourth terminal and the sixth terminal; while transferring asecond current from the fourth terminal to the sixth terminal via theconductor, measuring a second potential between the first terminal andthe seventh terminal; determining a resultant voltage generated by thefirst and second Hall effect sensors in response to the first and secondpotentials; and determining a value stored in the first and second Halleffect sensors responsive to the determined resultant voltage.
 5. Themethod according to claim 4, wherein at least one of the first andsecond Hall effect sensors generates normal Hall voltages.
 6. The methodaccording to claim 4, wherein least one of the first and second Halleffect sensors generates extraordinary Hall voltages.
 7. The methodaccording to claim 4, wherein the first Hall effect sensor comprises aplurality of ferromagnetic layers separated by a non-magnetic spacerlayer.
 8. The apparatus according to claim 1, wherein the first Halleffect sensor comprises a plurality of ferromagnetic layers separated bya non-magnetic spacer layer.